This invention relates generally to electronic clocks powered by an alternating current, and, more particularly, to auxiliary timing sources for these electronic clocks.
Electronic clocks, powered by alternating current (ac), have found widespread application and are embodied in such devices as alarm clocks, pool filter timers and lawn sprinkler and irrigation timers. Typically, the clock's timing source is either the alternating current's 50 or 60 Hz oscillation, which has a timing accuracy of approximately .+-.0.01%, or an internal crystal oscillator, which has an accuracy on the order of .+-.0.01 to 0.30%.
During a power failure, an auxiliary timing source is required. Typically, an auxiliary timing source includes a battery, a charger and some associated electronics. For example, the auxiliary timing source for an ac-powered clock might include a battery, a charger and an external oscillator. The external oscillator could be either a crystal oscillator, which is accurate but relatively costly, or a resistor-capacitor (RC) oscillator, which is inexpensive but inaccurate. Because the tolerances of the resistive and capacitive components are rather high, and because of temperature drift and voltage fluctuations, the RC oscillator has a relatively low accuracy, on the order of .+-.30%. The RC oscillator can be fine-tuned at the factory, but tuning is not an entirely satisfactory solution, since it entails the added cost of the tuning operation and a trimmer circuit. More importantly, the accuracy still cannot be reduced below about .+-.5%. The crystal-based time source already includes an oscillator, so a battery, and possibly a battery charger, are all that are required. However, as already mentioned, the crystal and its associated electronics are relatively costly.
The battery associated with the auxiliary timing source may also be used to provide the power required to maintain a volatile solid-state storage medium. The time of day and a set of timing instructions are typically stored in the storage medium. With such a configuration, the auxiliary timing source updates the time of day stored in the storage medium. However, when the battery becomes drained, the auxiliary timing source and the volatile storage medium will fail, losing the time of day and timing instructions. Upon restoration of the power, the time of day and the set of timing instructions will typically be reset to predetermined default values or to random values, resulting in erroneous commands to the device being controlled.
Accordingly, there exists a need for an inexpensive and accurate auxiliary timing source for generating the time of day during power failures and a storage medium that will maintain the time of day and the timing instructions in the event that the battery becomes drained.